document that we need a silence when setting up a new host

Closes: team#41827

howto/new-machine.md

@@ -279,6 +279,16 @@ above](#pre-requisites)! Some installers cover all of those steps, but most do n
 
         reboot
 
+ 10. if the machine was *not* installed from the Fabric installer (the
+     `host.install-hetzner-robot` task), schedule a silence for backup
+     alerts with:
+     
+        fab silence.create \
+          --comment="machine waiting for first backup" \
+          --matchers job=bacula \
+          --matchers alias=test-01.torproject.org \
+          --ends-at "in 2 days"
+
 At this point, the machine has a basic TPA setup. You will probably
 need to assign it a "role" in Puppet to get it to do anything.